Delayed coking process for producing anisotropic free-flowing shot coke

ABSTRACT

A delayed coking process wherein substantially all of the coke produced is free-flowing anisotropic shot coke. A coker feedstock, such as a vacuum residuum, is treated with an oxidizing agent, such as air, to increase the level of one or more of asphaltenes, polars, and organically bound oxygen groups. The oxidized feedstock is then heated to coking temperatures and passed to a coker drum for an effective amount of time to allow volatiles to evolve and to produce a substantially free-flowing anisotropic shot coke.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims benefit of U.S. provisional patentapplication serial No 60/336,778 filed Dec. 4, 2001.

FIELD OF THE INVENTION

[0002] The present invention relates to a delayed coking process whereinsubstantially all of the coke produced is free-flowing anisotropic shotcoke. A coker feedstock, such as a vacuum residuum, is treated with anoxidizing agent, such as air, to increase the level of one or more ofasphaltenes, polars, and organically bound oxygen groups. The oxidizedfeedstock is then heated to coking temperatures and passed to a cokerdrum for an effective amount of time to allow volatiles to evolve and toproduce a substantially free-flowing anisotropic shot coke.

DESCRIPTION OF RELATED ART

[0003] Delayed coking has been practiced for many years. The processbroadly involves thermal decomposition of petroleum residua (resids) toproduce gas, liquid streams of various boiling ranges, and coke. Delayedcoking of resids from heavy, and heavy sour (high sulfur) crude oils iscarried out primarily as a means of disposing of these low valuefeedstocks by converting part of the resids to more valuable liquid andgas products. Although the resulting coke is generally thought of as alow value by-product, it does have some value as a fuel (fuel grade),electrodes for aluminum manufacture (anode grade), etc.

[0004] In the delayed coking process, the feedstock is rapidly heated ina fired heater or tubular furnace. It is then passed to a coking drumthat is maintained at conditions under which coking occurs, generally attemperatures above about 400° C. under super-atmospheric pressures. Theheated residuum feed further decomposes in the coker drum to formvolatile components that are removed overhead and passed to afractionator leaving coke behind. When the coker drum is full of cokethe heated feed is switched to another drum and hydrocarbon vapors arepurged from the coke drum with steam. The drum is then quenched withwater to lower the temperature to about 200-300° F. after which thewater is drained. When the cooling step is complete, the drum is openedand the coke is removed after drilling and/or cutting using highvelocity water jets.

[0005] For example, a high speed, high impact water jet is used to cutthe coke from the drum. A hole is typically bored in the coke from waterjet nozzles located on a boring tool. Nozzles oriented horizontally onthe head of a cutting tool cut the coke from the drum. The coke removalprocess adds considerably to the throughput time of the process. Thatis, since it takes approximately 1 to 6 hours, typically about 3 hoursto drill-out and remove the resulting coke mass, the coker drumturn-around time and process costs are increased. Thus, it would bedesirable to produce a free-flowing coke in the coker drum that wouldnot require the expense and time associated with conventionalagglomerated coke mass removal.

[0006] Further, even though the coking drum may appear to be completelycooled, occasionally, a problem arises which is referred to in the artas a “hot drum.” This problem occurs when areas of the drum do notcompletely cool. This may be the result of a combination of morphologiesof coke in the drum resulting in a non-uniform drum. The drum maycontain a combination of more than one type of solid coke product, i.e.,needle coke, sponge coke and shot coke. BB-sized shot coke may coolfaster than another coke, such as large shot coke masses or sponge coke.Avoiding “hot drums” is another reason for producing predominantly shotcoke in a delayed coker.

[0007] Attempts have been made to produce predominantly, orsubstantially all of a single type of coke during delayed coking. Forexample, U.S. Pat. No. 5,258,115, which is incorporated herein byreference, teaches a delayed coking process wherein spent caustic isintroduced into a delayed coker feed, or into the coker drum itself, toproduce shot coke to help alleviate the hot drum problem. It alsoreduces cooling time.

[0008] Further, U.S. Pat. No. 3,960,704, which is also incorporatedherein by reference, teaches a delayed coking process wherein isotropiccoke is the product. Isotropic coke is coke that has thermal expansionapproximately equal along the three crystalline axes. This is achievedby air blowing a petroleum resid feedstock to a certain softening pointand running the coking process at relatively high recycle ratios andpreferably with a diluent oil.

[0009] Although delayed coking has been in commercial use for manyyears, there still remains a need in the art for improvements that canshorten the coke removal time.

SUMMARY OF THE INVENTION

[0010] In accordance with the present invention there is provided adelayed coking process wherein substantially all of the coke produced issubstantially free flowing anisotropic shot coke, which processcomprises:

[0011] a) contacting a vacuum resid feed with an oxidizing agent at atemperature from about 150° C. to about 375° C. for an effective amountof time to significantly increase the amount of asphaltenes andorganically bound oxygen in the resid;

[0012] b) heating said oxidized resid feed to a temperature effectivefor coking said feed;

[0013] c) charging said heated oxidized resid to a delayed coker drum ata pressure from about 15 to 50 psig for an effective amount of time toproduce volatiles and anisotropic substantially free-flowing shot coke;

[0014] d) removing at least a portion of said volatiles overhead; and

[0015] e) removing the anisotropic substantially free-flowing shot cokeproduct from the coker drum.

[0016] Also in accordance with the present invention there is provided adelayed coking process comprising:

[0017] a) contacting a vacuum resid with an oxidizing agent at atemperature from about 150° C. to about 375° C. for an effective amountof time to significantly increase the amount of asphaltenes and/orpolars and other organically bound oxygen groups in the resid;

[0018] b) heating said oxidized resid to a temperature effective forcoking said feed;

[0019] c) charging said heated oxidized resid to a delayed coker drum ata pressure from about 15 to 50 psig for an effective amount of time toproduce volatiles and a substantially free-flowing anisotropic shotcoke;

[0020] d) removing at least a portion of the volatiles overhead;

[0021] e) quenching the remaining hot coke bed with water;

[0022] f) removing the resulting anisotropic substantially free-flowingshot coke product from the coker drum.In one preferred embodiment of thepresent invention, the oxidizing agent is air.

[0023] In another preferred embodiment of the present invention acaustic can be added to the oxidized resid coker feedstock before,during, or after heating in the coker furnace.

BRIEF DESCRIPTION OF THE FIGURE

[0024]FIG. 1 hereof is a cross polarized light photomicrograph of cokeresulting from a San Joaquin Valley vacuum residuum that was not treatedwith an oxidizing agent prior to coking. The area of view is 170 micronsby 136 microns.

[0025]FIG. 2 hereof is a photomicrograph of coke resulting from a SanJoaquin Valley vacuum residuum that was treated with air for 3 hours ata temperature from 185° C. to 225° C. prior to coking. The area of viewis 170 microns by 136 microns.

DETAILED DESCRIPTION OF THE INVENTION

[0026] Feedstocks suitable for the delayed coking process of the presentinvention are petroleum vacuum residua. Such petroleum residua arefrequently obtained after removal of distillates from crude feedstocksunder vacuum and are characterized as being comprised of components oflarge molecular size and weight, generally containing: (a) asphaltenesand other high molecular weight aromatic structures that would inhibitthe rate of hydrotreating/hydrocracking and cause catalyst deactivation;(b) metal contaminants occurring naturally in the crude or resultingfrom prior treatment of the crude, which contaminants would tend todeactivate hydrotreating/hydrocracking catalysts and interfere withcatalyst regeneration; and (c) a relatively high content of sulfur andnitrogen compounds that give rise to objectionable quantities of SO₂,SO₃, and NO_(x) upon combustion of the petroleum residuum. Nitrogencompounds also have a tendency to deactivate catalytic crackingcatalysts. Typical examples of coker petroleum feedstocks which arecontemplated for use in the present invention, include residues from theatmospheric and vacuum distillation of petroleum crudes or theatmospheric or vacuum distillation of heavy oils, visbroken resids, tarsfrom deasphalting units or combinations of these materials. Atmosphericand vacuum topped heavy bitumens can also be employed. Typically, thesefeedstocks are high-boiling hydrocarbonaceous materials having a nominalinitial boiling point of about 538° C. or higher, an API gravity ofabout 20° or less, and a Conradson Carbon Residue content of about 0 to40 weight percent.

[0027] The coking process of the present invention is delayed coking,which is well known in the art. Generally, in the delayed cokingprocess, a bottoms fraction, such as a petroleum residuum chargestock ispumped to a heater at a pressure of about 50 to 550 psig, where it isheated to a temperature from about 480° C. to about 520° C. It is thendischarged into a vertically oriented insulated coker drum through aninlet at the base of the drum. Pressure in the drum is usuallyrelatively low, such as about 15 to 50 psig to allow volatiles to beremoved overhead. Typical operating temperatures of the drum will bebetween about 410° C. and 475° C. The hot feedstock thermally cracksover a period of time in the coker drum, liberating volatiles composedprimarily of hydrocarbon products, that continuously rise through thecoke mass and are collected overhead. The volatile products are sent toa coker fractionator for distillation and recovery of coker gases,gasoline, light gas oil, and heavy gas oil. At least a portion of theheavy coker gas oil present in the product stream introduced into thecoker fractionator is captured for recycle and combined with the freshfeed (coker feed component), thereby forming the coker heater or cokerfurnace charge.

[0028] There are generally three different types of solid delayed cokerproducts that have different values, appearances and properties. Theseare needle coke, sponge coke and shot coke. Needle coke is the highestquality of the three varieties. Needle coke, upon further thermaltreatment, has high conductivity and is used in electric arc steelproduction. It is relatively low in sulfur and metals and is producedfrom some of the higher quality coker feedstocks that include morearomatic feedstocks such as slurry and decant oils from catalyticcrackers and thermal cracking tars as opposed to the asphaltenes andresins.

[0029] Sponge coke, a lower quality coke, sometimes called “regularcoke,” is most often formed in refineries. Low quality refinery cokerfeedstocks having significant amounts of asphaltenes, heteroatoms andmetals produce this lower quality coke. If the sulfur and metals contentis low enough, sponge coke can be used for the manufacture of electrodesfor the aluminum industry. If the sulfur and metals content is too high,then the coke can be used as fuel. The name “sponge coke” comes from itsporous, sponge-like appearance. Conventional delayed coking processes,using the preferred vacuum resid feedstock of the present invention,will typically produce sponge coke, which is produced as an agglomeratedmass that needs an extensive removal process including drilling andwater-jet technology. This adds considerable time and costs to theprocess.

[0030] Shot coke has been considered the lowest quality coke because ithas the highest sulfur and metals content, the lowest electricalconductivity and is the most difficult to grind. The term “shot coke”comes from its shape which is similar to that of BB sized (about{fraction (1/16)} inch to ⅜ inch) balls. Shot coke, like the other typesof coke, has a tendency to agglomerate, especially in admixture withsponge coke, into larger masses, sometimes larger than a foot indiameter. This can cause refinery equipment and processing problems.Shot coke is usually made from the lowest quality high resin-asphaltenefeeds and makes a good high sulfur fuel source, particularly for use incement kilns and steel manufacture. The inventors hereof haveunexpectedly found that substantially free-flowing anisotropic shot cokecan be produced by first treating the residuum feedstock with anoxidizing agent to substantially increase the contents of itsasphaltene, and/or polars fractions, such as those containingorganically bound oxygen like ketones, carboxylic acids, etc. Theresiduum feed is subjected to the oxidizing agent, preferably air, ateffective temperatures, i.e., at temperatures that will encourage theformation of asphaltenes and organically bound oxygen groups to form.Such temperatures will typically be from about 150° C. to about 325° C.,preferably from about 185° C. to about 280° C., more preferably fromabout 185° C. to about 250° C. The oxidizing agent can be in anysuitable form including gas, liquid or solid. Non-limiting examples ofoxidizing agents that can be used in the practice of the presentinvention include air, oxygen, ozone, hydrogen peroxide, organicperoxides, hydroperoxides, inorganic peracids, inorganic oxides andperoxides and salts of oxides, sulfuric acid, and nitric acid. Preferredis air. It is to be understood that after the resid is treated with theoxidizing agent, a caustic, preferably a spent caustic, may optionallybe added. The spent caustic can also be added before, during, or afterthe oxidized resid is passed to the coker furnace and heated to cokingtemperatures. The caustic will be an alkali-metal material preferably aspent caustic soda and/or potash stream that is typically used invarious refinery processes. Such spent caustic streams typically containone or more of sodium and potassium, sulfur, and other wastes, includingorganic contaminants that vary depending on the hydrocarbon source butcan be organic acids, dissolved hydrocarbons, phenols, naphthenic acids,and salts of organic acids. The spent caustic stream will usually have arelatively high water content, typically about 50 wt. % to 95 wt. %water, more typically from about 65 wt. % to about 80 wt. %.

[0031] The precise conditions at which the residuum feedstock is treatedwith the oxidizing agent is feed dependent. That is, the conditions atwhich the feed is treated with the oxidizing agent is dependent on thecomposition and properties of the feed to be coked. These conditions canbe determined by one having ordinary skill in the art without undueexperimentation. Several runs are made with a particular feed atdifferent oxidizing times and temperatures followed by coking. Theresulting coke is then analyzed by use of a microcarbon test procedureand microscopy as set forth in the examples hereto. The desired cokemorphology that will produce substantially free-flowing coke is a cokemicrostructure of discrete micro-domains having an average size of about1 to 10 μm, preferably from about 1 to 5 μm, somewhat like a mosaic(FIG. 2 hereof). Coke microstructure that represents coke that is notfree-flowing anisotropic shot coke is the microstructure represented inFIG. 1 hereof that show a coke microstructure that is composedsubstantially of non-discrete, or substantially large flow domains up toabout 60 μm or greater in size, typically from about 10 to 60 μm.

[0032] U.S. Pat. No. 3,960,704 which is incorporated herein byreference, teaches delayed coking wherein a resid feedstock is air blownto a target softening point. The air blown feed is then passed todelayed coking process that is operated at conditions that will favorthe formation of isotropic coke. That is, coke particles havingsubstantially equal thermal expansion properties along the three majorcrystalline axes. This '704 patent requires relatively high recycleratios and an additional amount of oil as a diluent to produce apellet-type isotropic coke. For example, the recycle ratio of this '704patent is from about 1 to 5. This correlates to 100% to 500% recyclebased on fresh feed. Although up to about 15% recycle can be used in thepractice of the present invention it is preferred that no recycle beused. The presently claimed delayed coking process does not produceisotropic pellet-type coke—it produces substantially free-flowinganisotropic shot coke. Also, the shot coke that results from thepractice of the present invention can be easily removed from the cokerdrum without drilling or the use of water-jet cutting technology. Whileshot coke has been produced by conventional methods it is typicallyagglomerated to such a degree that water-jet technology is needed forits removal.

[0033] It is important to the practice of the present invention that theresid feedstock be first treated with an oxidizing agent tosubstantially increase its level of asphaltenes, polars, and organicallybound oxygen groups that encourages the formation of anisotropicsubstantially free-flowing shot coke. It is also important to thepractice of the present invention that the coker drum be kept atrelatively low pressures in order to allow as much of the evolvingvolatiles to be collected overhead. This helps prevent agglomeration ofthe resulting shot coke. The recycle ratio, that is the volumetric ratioof furnace charge (vacuum resid plus recycle oil) to fresh feed to thecontinuous delayed coker operation should also be kept as low aspossible. The use of recycle ratio for delayed coking is taught in moredetail in U.S. Pat. No. 3,116,231 which is incorporated herein byreference.

[0034] The present invention will be better understood by reference tothe following examples that are presented for illustrative purposes onlyand are not to be taken as limiting the invention in any way.

EXAMPLES

[0035] General Procedure: Approximately 180 g each of five differentpetroleum residua were added to a 500 cc round bottom flask equippedwith a Therm-O-Watch controller, a mechanical blade stirrer, and acondenser attached to a Dean-Stark trap to recover any light ends andwater generated during the reaction. The residuum was heated to 180° C.at which time air was introduced into the hot residuum feed under itssurface by means of a sparger tube. The temperature was raised andcontrolled to between 220° to 230° C. and the flow rate of air wascontrolled at 0.675 ft³/hr for three hours or as required depending onthe desired degree of oxidation. The sparger tube was removed after thedesired time and the flask was allowed to cool to room temperature.

[0036] Deasphalting Procedure: A mixture of fresh or oxidized coker feedand n-heptane were added to a 250 cc round bottom flask in a ratio of 1part feed to 8 parts n-heptane and allowed to stir for 16 hours at roomtemperature. The mixture was then filtered through a coarse Buchnerfunnel to separate the precipitated asphaltenes. The solids were driedin a vacuum oven at 100° C. overnight. The heptane was evaporated fromthe oil/heptane mixture to recover the deasphalted oil. The amount ofasphaltenes produced from the oxidized feed was compared to the amountgenerated from the starting residuum under the same deasphaltingprocedure. The results are presented in the following table: TABLE 1Enhancements of Feed Properties by Air Oxidation Favors Formation ofAnisotropic Loose Shot Coke San Joaquin LA Sweet Heavy Midwest ValleyOxidized Maya Canadian Raw Oxidized Raw Oxidized Raw (6 hr) Raw OxidizedRaw Oxidized Asphaltenes, 8.9 27.0 13.6 37.8 0 31.7 40.9 41.0 19.4 28.3wt %

[0037] Microcarbon residue tests were performed on the above feeds togenerate cokes to be evaluated by microscopy. The following is theprocedure used for the microcarbon tests: Heating Profile Time (min) N2Flow (cc/min) Heat from room temp to 10 66 100° C. Heat from 100° C. to30 66/19.5 300° C. then to 500° C. Hold at 500° C. 15 19.5 Cool to roomtemp 40 19.5

[0038]FIGS. 1 and 2 are cross polarized light photomicrographs showingthe microstructure of the resulting coke from a San Joaquin Valleyresiduum for both the untreated residuum and the residuum treated withair in accordance with the above procedure. The viewing area for both is170 microns by 136 microns. The untreated residuum resulted in a cokewith a microstructure that was not discrete fine domains. The domainswere relatively large (10-30 μm) flow domains. This indicates that amixture of shot coke and sponge coke will be produced in the coker drumof a delayed coker. The microstructure (FIG. 2) of the resulting cokefrom the residuum sample that was first air oxidized shows relativelyfine (2-5 μm) discrete fine domains indicating that free-flowing shotcoke will be produced in the coker drum of a delayed coker. Followingthe same procedure, the following changes in flow domain sizes wereobserved: a Midwest Vacuum Resid (10-50 μm to 2-3 μm), a Louisiana SweetVacuum Resid (20-60 μm to 2 to 5 μm) in six hours, a Maya Vacuum Resid(2-10 μm-no change), and a Heavy Canadian Vacuum Resid (10-20 μm to 2-10μm).

1. A delayed coking process wherein substantially all of the cokeproduced is substantially free-flowing anisotropic shot coke, whichprocesses comprises: a) contacting a vacuum residuum feed with anoxidizing agent at a temperature from about 150° C. to about 325° C. foran effective amount of time to significantly increase the amount of oneor more of asphaltenes, polars, and organically bound oxygen groups inthe resid; b) heating said oxidized resid feed to a temperatureeffective for coking said feed; c) charging said heated oxidized residto a delayed coker drum at a pressure from about 15 to 50 psig for aneffective amount of time to produce volatiles and anisotropicsubstantially free-flowing shot coke; d) removing at least a portion ofsaid volatiles overhead; and e) removing the product anisotropicsubstantially free-flowing shot coke from the coker drum.
 2. The processof claim 1 wherein the oxidizing agent is selected from air, oxygen,ozone, hydrogen peroxide, organic peroxides, hydroperoxides, inorganicperacids, inorganic oxides and peroxides and salts of oxides, sulfuricacid, and nitric acid.
 3. The process of claim 2 wherein the oxidizingagent is selected from air, oxygen, and ozone.
 4. The process of claim 3wherein the oxidizing agent is air.
 5. The process of claim 1 whereinthe temperature at which the residuum is treated with the oxidizingagent is from about 185° C. to about 280° C.
 6. The process of claim 1wherein an aqueous caustic is added to the residuum before, during, orafter being heated to coking temperatures.
 7. The process of claim 6wherein an aqueous caustic is added to the residuum after being heatedto coking temperatures.
 8. The process of claim 1 wherein the particlesize of the shot coke is from about {fraction (1/16)} to ⅜ inch.
 9. Theprocess of claim 1 wherein the microstructure of the resultingsubstantially free-flowing anisotropic coke is characterized as beingcomprised of substantially discrete domains from about 1 to 10 μm inaverage size.
 10. A delayed coking process comprising: a) contacting avacuum residuum with an effective amount of air at a temperature fromabout 150° C. to about 325° C. for an effective amount of time tosignificantly increase the amount of one or more of asphaltenes, polars,and organically bound oxygen in the residuum; b) heating said oxidizedresiduum to a temperature effective for coking said feed; c) chargingsaid heated oxidized residuum to a delayed coker drum at a pressure fromabout 15 to 50 psig for an effective amount of time to produce volatilesand a substantially free-flowing anisotropic shot coke; d) removing atleast a portion of the volatiles overhead; e) quenching the remaininghot coke bed with water; f) removing the resulting anisotropicsubstantially free-flowing shot coke product from the coker drum. 11.The process of claim 10 wherein the temperature at which the residuum istreated with the oxidizing agent is from about 185° C. to about 280° C.12. The process of claim 10 wherein an aqueous caustic is added to theresiduum before, during, or after being heated to coking temperatures.13. The process of claim 12 wherein an aqueous caustic is added to theresiduum after being heated to coking temperatures.
 14. The process ofclaim 10 wherein the particle size of the shot coke is from about{fraction (1/16)} to ⅜ inch.
 15. The process of claim 10 wherein themicrostructure of the resulting substantially free-flowing anisotropiccoke is characterized as being comprised of substantially discretedomains having an average size of about 1 to 10 μm.